[time-nuts] GPSDO unclear situation

paul swed paulswedb at gmail.com
Wed Jan 27 22:16:02 UTC 2016

Attila I am pretty sure Anton is saying compare it to the H Maser and it
simply doesn't in any way. I think we are speaking 1X10-11 compared to 1
X10-14th or so.
Thats why I say forget all of the GPS stuff and go for an output from the
maser to home. Now that would be a really fun project. Thinking RF. But
need to stop going down that rabbit hole as I am lacking a good H maser.
Sorry for the detour back to Antons thread.

On Wed, Jan 27, 2016 at 4:26 PM, Attila Kinali <attila at kinali.ch> wrote:

> On Wed, 27 Jan 2016 12:17:32 +0000 (UTC)
> Anton Moehammad via time-nuts <time-nuts at febo.com> wrote:
> > Hi All,
> > My GPSDO based from VE2ZAZ design but I have a few question I do
> > not clearly understand about GPSDO, like :
> >  1.How important the quality of 1 PPS signal based from jitter etc what
> the
> > difference I will get between timing purpose GPS module like Ublox 6T or
> > trimble resolution T with a position GPS module like ublox 6M or other in
> > term of stability, accuracy and phase noise.
> I think the best improvement you can make is by not using a VE2ZAZ style
> GPSDO :-) The design uses an FLL, which results in a slight frequency
> error. If you change it to a PLL, you will only get a slight phase error.
> Next, PPS resolution will be limted by the uC internal counter.
> Ie your resolution is in the 10-50ns range. You will need to get this
> down to <1ns and apply saw-tooth correction if you want to see any
> difference between a standard GPS receiver or a timing GPS receiver.
> As there is no LEA-6M I assume you mean the LEA-M8T. The Trimble
> Resolution,
> the LEA-6T and the LEA-M8T are all timing receivers. The differences
> between
> those are small. Smaller than all the other error contributions due to your
> device design and your antenna position. Hence I wouldn't change the
> receiver, once you have found one that fits your need.
> >  2. I understand short time stability from OCXO is better than GPSDO ?
> > can someone give me a clue what the time constant need for OCXO control
> > (in my application 30 second) is that enough ?
> This highly depends on your OCXO, your GPSDO design and the performance
> the GPS receiver delivers. There have been many papers written on this
> topic and you need a fair bit of understanding on control theory to apply
> them correctly. Usual time constants for the control loop are between
> one second and a couple of minutes.
> > 3. I have few Rubidium oscillator and I read somewhere (I believe in
> > web) that GPSDO with rubidium osc is hard to beat, in what way ? short
> time,
> > long time, phase noise.
> The reason is because you can integrate over days using an Rb local
> oscillator instead of just seconds or minutes. This filters out all
> periodic perturbations induced by the GPS system.
> The phase noise might or might not be better though. This highly depends
> on how much care was given to the Rb's quartz oscillator output.
> E.g. the ubiquitus FE-5680A has a very noisy output, worse than
> a simple crystal oscillator.
> > My goal is "simple" I have access to national time reference lab
> > ( an active Maser H ) I want to compare my reference to them off course
> > there is no way I have same stability but anything not so "embrace" is
> fine
> > to me
> I am not sure I understand what you mean here. Do you want to compare
> your GPSDO to their H-maser? Or do you want to compare some local
> reference you have using the GPSDO as transfer standard?
>                         Attila Kinali
> --
> Reading can seriously damage your ignorance.
>                 -- unknown
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